The present invention is directed to an analyzer for automatically measuring the vapor-liquid ratio of a fluid, and particularly to an on-line analyzer that can be used to control the addition of volatile liquids in blended gasoline products.
In the production of blended gasoline products, various additives are blended with the refined gasoline stock to impart certain desired properties to the final product. For example, these additives can include corrosion inhibitors and anti-knock preparations. Another component which is typically added to gasoline products is a liquid having high volatility, such as butane or natural gasoline. It is necessary to impart a certain degree of volatility to the blended gasoline product, to improve the properties that affect the cold starting of engines. In addition, it is further desirable to add as much butane to the gasoline product as feasible. Since butane is much less expensive than gasoline, the more butane that is added to the blended gasoline contributes to a greater reduction in the overall cost of the final product.
However, the amount of butane that can be added to the blended gasoline product is limited due to its high volatility. If the volatility of the blended gasoline is too high, vapor lock can occur at elevated temperatures, such as those that are encountered under the hood of a vehicle. When vapor lock occurs, the supply of liquid fuel to the engine is blocked and the engine stalls until the temperature can be reduced and the evaporated gasoline condenses.
During blending, the vapor-liquid ratio (V/L) of a blended gasoline product is monitored to indicate the amount of butane that can be safely added to the product. Basically, the vapor-liquid ratio is a measurement of the relative volumes of vapor and liquid of the product that exist under equilibrium conditions at a specified temperature and pressure. In gasoline products, a vapor-liquid ratio of twenty is generally accepted as the ratio where vapor lock begins. In the blending of gasoline, a controlling parameter is not so much the actual vapor-liquid ratio of the gasoline, but rather the temperature at which a vapor-liquid ratio of twenty exists for a specified pressure. This temperature will determine the climatic conditions under which the gasoline will be suitable for use. Thus, the gasoline producer is typically concerned with blending the gasoline so that it has a specified minimum vapor-liquid ratio temperature, which temperature is associated with the particular geographic area and the time of year in which the gasoline is to be distributed.
In the past, the vapor-liquid ratio of a blended gasoline product was estimated from correlations based on the Reid vapor pressure of the gasoline and other test parameters established through standard test methods approved by the American National Standards Institute. However, there are a number of limitations associated with such estimates. Every estimate includes an unknown correlation error. The magnitude of this correlation error is dependent upon the composition of the blended product, and therefore varies almost continuously during blending since the composition of the product changes. In addition, every estimate includes the errors that are generated from the separate measurements of the Reid vapor pressure and the ASTM test parameters. Errors in the measurement of the Reid vapor pressure in particular can be large and hard to evaluate with particularity.
Accordingly, there is a need for an analyzer that can directly measure the vapor-liquid ratio of a liquid at a temperature and pressure of interest. One known type of analyzer that performs such a function is disclosed in U.S. Pat. No. 3,491,585. The analyzer disclosed in this patent operates in a continuous fashion, wherein a stream of a liquid to be tested is heated to a temperature so that the volatile constituents of the liquid become vaporized. The liquid and vapor components are then separated, and their respective flow rates are compared to one another to determine the vapor-liquid ratio of the liquid.
While an analyzer of the type disclosed in the '585 patent is commercially available, it is not known to be in wide use. Part of the reason for its lack of acceptance is due to its relatively high price. Furthermore, it is a rather complex system and accordingly is difficult to keep operational at a satisfactory level. In addition, it provides data in the form of the absolute vapor-liquid ratio, rather than the temperature for a ratio equal to 20, which is a parameter that is better understood and utilized by blenders.
Other continuously operating types of vapor-liquid ratio measuring systems are disclosed in U.S. Pat. Nos. 3,276,460; 3,686,924; 3,735,634 and 3,813,925. As far as is known, none of these systems are presently in use. It appears that they suffer from the same complexity as that of the '585 patent, and for this reason they may not be suited for practical use.
In addition to the continuously operating analyzers, vapor-liquid ratio measuring systems that operate in batch mode are also known. Representative of U.S. Patents disclosing these type of systems are U.S. Pat. Nos. 3,145,561; 3,528,439 and 3,528,440. It appears that these systems also are relatively complex, and it is believed that none of them are being used at this time.